Proc. Fla. State Hort. Soc. 53:155-158. 1940.
SPRAYING
EXPERIMENTS
ANTHRACNOSE
FOR
CONTROL
OF
AVOCADO
GEO. D. RUEHLE
Sub-Tropical Experiment Station, Homestead, Florida
The Haden variety makes up fully 90 percent of the plantings of named varieties of
mango grown commercially in Florida. This fruit finds ready acceptance on the markets
because of its excellent flavor, beautiful appearance, and good carrying quality in transit
and storage. The shy-bearing habit of the Haden has been far from satisfactory to
commercial growers, however.
We are unfamiliar, as yet, with all the conditions necessary for a proper setting of the
Haden but a number of factors contribute to its lack of fruitfulness. Such adverse
weather conditions as low temperatures and relatively high humidity during the flowering
season operate in some instances to prevent fruit setting. There is considerable
evidence that lack of opportunity for cross pollination with other varieties is highly
important as a factor limiting fruit setting1. It is probable that lack of pollination is
responsible for the temporary setting of large numbers of seedless fruits, which usually
drop prematurely 01 fail to attain commercial size and quality if carried to maturity.
Infection by the anthracnose, Colletotrichum gleosporioides Penz., has long been
considered a factor of primary importance in causing unfruitfulness of mangos in
Florida. The various manifestations of the disease include a leaf-spot, blossom blight,
fruit russeting, and fruit rot. Attacks on the bloom and setting fruits are often severe. The
injury caused by the disease is closely dependent upon humidity, the prevalence of
rains or heavy dews greatly increasing its incidence.
The causal organism is one of the most widely distributed pathogenic fungi in Florida. It
causes the well-known wither-tip of citrus, a ripe rot of avocado and papaya fruits, and
is pathogenic on at least several other fruits. It grows and sporulates abundantly during
wet weather on the dead twigs and leaves of many plants, including the mango,
avocado, and Citrus species. It seems likely that the possibilities for infection are very
great at all times, and all that is needed is a favorable period of moisture to produce the
disease in abundance.
Anthracnose can be controlled by spraying with bordeaux mixture. Stevens2 has worked
out a spray schedule which includes five applications of 8-8-100 bordeaux mixture timed
as follows: (1) dormant spray; (2) open bloom spray, when two-thirds or more of the
flowers are open; (3) three weeks after No. 2, when most of the fruits have set; (4) one
month after No. 3; and (5) one month after No. 4. Blossom infection and anthracnose of
the fruit was controlled fairly well by this schedule in average seasons, provided the fruit
set normally. Even in dry seasons, when little anthracnose was present, the percentage
of first grade fruit was increased, and sprayed fruit carried better in transit and storage
than unsprayed fruit.
Bordeaux mixture made according to the 8-8-100 formula is a stronger fungicide than
necessary to control attacks of anthracnose on the avocado. Heavy bordeaux residues
retard the growth of such plants as the tomato and the potato and probably of other
plants as well. Heavy spray residues non-toxic to scale insects and spider mites
generally tend to markedly increase such pests. The so-called "insoluble" or "neutral"
copper compounds are less objectionable than bordeaux mixture in these respects.
There appear to be no spraying experiments reported on the mango with these neutral
copper materials, with the exception of limited tests by Stevens with tri-basic copper
sulfate in 1935. Bordeaux mixtures weaker than the 8-8-100 formula apparently have
not been tried.
Laboratory tests indicated the 3-3-100 bordeaux mixture and certain of the neutral
copper compounds were sufficiently toxic, when sprayed on glass slides, to prevent all
germination of Collectotrichum spores. Field experiments to test these fungicides for
control of mango anthracnose were started in the 1937-38 season.
Spraying Experiment
The experiments were conducted at the Coral Reef Nurseries, near Cutler, in 10acre
grove of Haden mangos. The sprays were applied in a uniform manner with power
machinery at 400 to 500 pounds pressure.
An individual plot consisted of a single row of 22 trees. Each treatment was tested in
duplicate plots. The various fungicides were compared in schedules which included a
dormant spray in one case and no dormant spray in another. The dormant spray was
applied when the buds were just beginning to swell. The second spraying in these plots,
and the first spraying in plots not receiving dormant spray, was made when the first
bloom spikes were well opened but before individual flowers were open (Plate 1).
Successive sprayings were made at approximately 1, 3, 7, and 11 weeks after this first
bloom spike application.
The entire tree was covered thoroughly at the dormant and first bloom spike
applications, but thereafter attention was directed to covering only the bloom spikes and
fruits thoroughly. Spreaders known to be efficient on other types of foliage were
incorporated with the spray mixture unless already added in the manufacturing process,
as is the case with some of the proprietary copper fungicides.
The mature fruits were examined as they were harvested, and were graded according
to the severity of infection into three classes, as follows: (1) free of anthracnose; (2)
stained or russetted; and (3) those showing active decay spots. Practically all of the
fruits which set from the early or main bloom were examined from each plot.
The spray schedule and the results are shown in Table 1.
Discussion
The results indicate that it is unnecessary to apply a dormant spray. Considerable
spraying had been done in previous years in the grove where the experiments were
conducted, and there was comparatively little anthracnose on the old foliage. A dormant
spray causes little reduction of infection on bloom spikes and fruit under these
conditions. It appears to be more practical to delay the first spraying until the bloom
spikes have opened, but before individual flowers have opened.
The number of bloom applications necessary will depend largely upon the weather
conditions during the blossoming period, and the uniformity with which the flower spikes
open. They do not all open at the same tore in the main bloom, and in some seasons,
lack of uniformity of opening is more evident than in others. It may be necessary, in
some seasons, to apply only two bloom sprays, while in others, three or four bloom
sprays may be desirable.
The results show that 4-4-100 bordeaux is as effective as the 6-6-100 formula, and it is
probable that stronger bordeaux mixtures would give no better results. The results
obtained in the 1937-38 season indicate that 3-3-100 bordeaux is not strong enough to
give good control.
Red cuprous oxide gave the best results of the three insoluble copper sprays tested.
Sufficient tests have not been made with this spray to determine what concentration of
the material is most practical to use. For the present, 3 pounds to 100 gallons of water
appears to give as good control of anthracnose as 6-6-100 bordeaux mixture, and may
be substituted in the spray schedule for bordeaux. Tri-basic copper sulfate 3-100 gave
fairly good results in the 1938-39 season and is worthy of further trial. Copper silicate at
3 pounds to 100 gallons of water is apparently not strong enough to give good control. It
is possible that better control would be obtained with this spray if its concentration were
increased.
Scale counts from grapefruit, orange, and avocado trees sprayed with various copper
sprays proved that scale insects increase at a faster rate after bordeaux mixture than
after insoluble copper sprays. Field observations indicate that a similar situation occurs
when the same sprays are used on the mango. This may be an important point for
consideration in determining which fungicide should be used.
Summary
Mango anthracnose on the blossom spikes and fruit of the main bloom may be
controlled to a large extent by thorough applications of copper fungicides properly
timed.
Dormant sprays do not seem to be necessary in commercial groves which have been
sprayed annually. The first application should be made when the bloom spikes are well
opened but before the individual flowers have opened.
One or two additional bloom sprays are necessary to control blossom spike and early
fruit infection, the number depending upon the weather and the uniformity of opening of
the flower spikes.
Usually at least two post-bloom sprays are necessary to prevent serious late infection of
the fruit.
It is not necessary to use bordeaux stronger than the 6-6-100 formula, and even 4-4100
bordeaux seems to be strong enough.
Red cuprous oxide 3-100 may be substituted for bordeaux mixture with about equal
results. Tri-basic copper sulfate 3-100 has given effective control of anthracnose in
experiments of one season.
Scale insects increase at a faster rate on trees sprayed with bordeaux mixture than on
trees sprayed with red cuprous oxide or tri-basic copper sulfate.
1 Ruehle, Geo. D., and S. J. Lynch. Observations and the influence of other varieties on
the yield of Haden mangos in mixed plantings. Florida Grower for July, 1939.
2 Stevens, H. E. Control of mango blossomblight and anthracnose. Proc. Florida State
Hort. Soc. for 1936: 125-130.